Oxygen ion transport type thermistors

ABSTRACT

An oxygen ion transport type thermistor comprising an oxygen ion type solid solution consisting of 50-95 mol percent of an oxide of tetravalent metals and 5-50 mol percent of oxide(s) of divalent and/or trivalent metals and two metal lead wires embedded in said solid solution, in which said two metal wires are embedded and held oppositely in substantially the center line in the thickness direction of said solid solution, parallel and adequately space and that the porosity of said oxygen ion type solid solution is 7-55 percent.

United States Patent Ushida et al.

OXYGEN ION TRANSPORT TYPE THERNHSTORS Inventors: Yoshiro Ushida,Toyoake; Shinji Nishio, Komaki, both of Japan Assignee: NGK Spark PlugCo., Ltd., Nagoya, Japan Filed: June 14, 1974 Appl. No.: 479,472

Foreign Application Priority Data June 21, I973 Japan 48-70493 US.Cl.... 338/22 R; 252/520; 252/521 Int. Cl. HOIC 7/04 Field of Search338/22, 23, 25; 29/612;

References Cited UNITED STATES PATENTS 3/1961 lchikawa 338/22 R Oct. 14,1975 2,985,700 5/1961 Johnston 338/22 R 3,044,968 7/1962 lchikawa 338/22R 3,377,561 4/1968 Sauer 338/22 R 3,786,390 1/1974 Kristen 338/22 RPrimary ExaminerC. L. Albritton Attorney, Agent, or FirmStevens, Davis,Miller & Mosher ABSI'RACT 2 Claims, 2 Drawing Figures U.S. Patent Oct.14, 1975 F l6. l5

' F/GJA OXYGEN ION TRANSPORT TYPE THVERMISTORS The present inventionrelates to thermistors.

Oxygen ion transport type sintered oxide solid solutions having a stablecrystal structure until a high temperature zone consisting of 50-95 molpercent of an oxide of tetravalent metals such as ZrO CeO l-ifO and ThOand 5-5O mol percent of oxide(s) of divalent alkaline earth metalsand/or trivalent rare earth metals, such as CaO, MgO, SrO, La O Y O Yb OSC O Gd O and Nd O decrease rapidly the electrical resistance at atemperature'of 400-l ,200C and have excellent properties as a hightemperature resistor of a thermistor. Such a solid solution is generallycomposed of polycrystalline fluorite structures, which have oxygenvacancies to preserve lattice neutrality. Accordingly, these oxidesintered bodies have been used for parts for measuring or controllingtemperature of a high temperature furnace and an apparatus for purifyingan exhaust gas of an internal combustion engine. Such ceramic resistorsare referred to as oxygen transport type thermistors, because theelectrical conductivity is shown to be due to the transport of oxygenion of the oxide in the solid solution constituting the resistor.Heretofore, the electrodes of such oxygen ion transport type thermistorsare provided by coating a platinum paste on both parallel main surfacesof the ceramic resistor fired in a disc form and arranging platinumwires to form lead wires thereon and bonding said wires to the resistorswith the above described platinum paste and firing said paste at atemperature of l ,0OO1,500C, but the adhesion at the portions where thelead wires get out of the ceramic resistor, is weak and further theworking steps are many and troublesome.

The adhesion at the bonded portions of the lead wires is degraded withthe raising of the temperature owing to softening of vitreous componentsin the platinum paste at a high temperature. Accordingly, it isimpossible at a high temperature, for example, higher than l,0()C toincrease the bonding strength of the lead wires even by such oxidationresistant metal paste. Furthermore, the above described coated metalpaste requires a moderate vapor permeability in order to make smooth thereceiving and supply of oxygen ion in the atmosphere to be convertedinto electricity at the contact portion of the electrodes with theresistor, so that a thick coating exceeding a certain degree cannot beeffected. Accordingly, the adhesion cannot be essentially improved.

While, the oxygen ion transport type thermistors according to thepresent invention which aim to obviate the above described drawbacks,are characterized in that as shown in FIG. 1, a molding is effected insuch a manner that two fine metal wires 2, 2 composed of metals having ahigher oxidation resistance, such as platinum or platinum-rhodium alloyare embedded in a resistor matrix leaving a space in parallel and themolded assembly is fired at an adequate temperature to form electrodesand lead wires which are embedded and held in a ceramic resistor 1 ofthe thermistor and that the porosity of the above described ceramicresistor 1 is made to be 755 percent, preferably l-35 percent, moreparticularly 25 percent.

The conventional oxygen ion transport type thermistor in which theelectrodes are provided on the surfaces 2 of the ceramic resistor, arevery tightly sintered and the porosity is usually less than 2 percent.

The reason why the lower limit of the porosity of the ceramic resistorof the thermistor according to the present invention is defined to be 7percent is as follows. When the oxygen ion transport type thermistor issupplied with a given direct current voltage of about 12V usually usedbetween the electrodes, the oxygen molecule is absorbed from theatmosphere near the cathode in order that the oxygen ion 0 whichtransports in the ceramic resistor and is concerned in the electrictransmission mechanism, is supplied at the cathode side as /2O +2e 0 andsaid oxygen molecule is discharged into the atmosphere near the anode inorder that the above described oxygen ion is taken out at the anode sideas 0 /2O +2e. But in the thermistor of the present invention, both theelectrodes are provided by embedding the fine metal wires in the ceramicresistor, so that the contact area of the electrodes to the resistor issmall and consequently if the porosity in the resistor is less than 7percent, the diffusion movement of the oxygen molecule which is effectedthrough pores in the resistor between air in the pores near theelectrodes and the ambient atmosphere, becomes not free and particularlywhen the oxygen to be supplied at the cathode side is deficient, theoxygen ion in the crystal structure of the sintered body of oxides isused for the electric transmission after the electric current flows fora given time under a high temperature condition and the oxides arereduced and blackened and the resistor becomes a semiconductor having avery small resistance and loses the normal function. Accordingly, inorder to prevent such a fact and to ensure the stable circulationmechanism of oxygen during use for a long time, the porosity must be notless than 7 percent. On the other hand, the above described upper limitof the porosity is defined for on the following reason. When theporosity is more than 55 percent, the variation of the electricresistance with the lapse of time in the ceramic resistor not onlybecomes larger but also the mechanical strength lowers and theelectrodes are readily removed and cracks and breaks are apt to becaused. When the porosity is too large, the contact resistance betweenthe electrodes and the resistor is large and further the adhesion isgradually lowered owing to the ion conversion at the electrode portions.

The present invention will be explained in more detail.

For a better understanding of the invention, reference is taken to theaccompanying drawings, wherein:

FIG. 1A is a plan view of the oxygen ion transport type thermistoraccording to the present invention; and

FIG. 1B is a side view of said thermistor.

The following examples are given for the purpose of illustration of thisinvention and are not intended as limitations thereof.

EXAMPLE 1 Each of mixtures of powdery ZrO and CaCO having compositionsas shown in the following Tables 1-3 (83 mol percent of ZrO +l7 molpercent of CaO(CaCO 87 mol percent of ZrO +l3 mol percent of CaO(- CaCOmol percent of ZrO +25 mol percent of CaO(CaCO was added with 4 percentby weight of an emulsion consisting of equal weight amounts of stearicacid and water as a binder and pulverized in a wet process in a trommelfor 20 hours. Each of the resulting 3 powdery mixtures was filled in amold and two 0.4 mm platinum wires were held in the center portion andthe filled mixture in the mold was pressed at a pressure of 2,000 Kg/cmand the molded mixture was fired at vari- 4 cording to the presentinvention did not cause such a variation and further even after the testwas continued for l0,000 hours, the resistance increases only slightlyand satisfactory durability was attained. However,

ous temperatures of about 1,400l ,800C in air to 5 when the porosityexceeds 55 percent, the adhesion produce oxygen ion transport typedisc-shaped therm- (pulling strength) of the platinum electrodes isinsuffiistors having different porosities as shown in the atcient andthere is problem in practical use. The test retached drawing, which havean outer diameter of about sults are shown in the following Tables 1-3showing 3.8 mm and a thickness of about 1.4 mm and in which data fromthe. Process for testing the tension strength a pair of platinum wires2, 2 are embedded and fixed in 10 of the electrodes. parallel at thecenter line of the thickness direction at a The two electrode wires areseparately secured with spacing of about 2 mm. The resulting thermistorwas clips at a distance of 3 mm'from the ends of the ceramic appliedwith DC12V at 800C between the two platiresistor and pulled in such adirection that the two elecnum electrodes for 1,000 hours. This testshowed that trode wires are diverged, through a spring system tenin thethermistors having the porosity of less than 7 per- 15 sion gauge fixedbetween the two clips and the strength cent, the vicinity of theelectrodes became grey or when the electrode wires are removed from theabove black and the run away phenomenon occurred, while describedresistor, is determined and is shown by an avthe thermistors having theporosity of 7-55 percent acerage value of 10 samples.

Table l 83 mol7r of ZrO +l7 mol7 of CaO(CaCO Resistance 800C, 12V

Tension Firing After After After strength tempcri 100 1,000 between Testature Porosity Initial hours hours hours electrodes N (K (K g) 1 1,45065 26 17 16 120 0.2 2 1,480 58 13 9.3 11 21 0.5 3 1,485 55 8.7 7.6 7.013 0.6 4 1,500 52 7.2 7.0 6.0 10 0.7 5 1,520 45 5.1 4.6 4.2 5.7 1.0 61,535 40 4.1 3.8 4.4 1.2 7 1,550 35 3.5 3.5 3.5 3.8 1.4 8 1,570 30 3.03.3 3.3 3.7 1.6 9 1,600 25 2.8 3.1 3.5 3.7 1.8 10 1.650 20 2.6 3.0 3.23.7 2.1 11 1,665 2.5 3.0 3.2 3.6 2.1 12 1,680 10 2.5 2.9 3.2 3.3 2.2 131,700 6.3 2.5 1.4 0.2 run 2.2

away 14 1,730 4.3 1.4 run 2.2

away

Table 2 87 mol% of ZrO +l3 mol% of CaO(CaCO Resistance 800C. 12V

Tension Firing After After After strength temperl0 100 1,000 betweenTes't uture Porosity Initial hours hours hours electrodes No. ("C) (Zr)1 K0 (K0) (K0) (K9 (Kg) 27 1,780 6 1.5 0.8 run 2.2

away 28 1,800 4 1.0 run 2.2

Table 3 75 mol% of ZrO +25 mol% of CaO(CaCO Resistance 800C, 12V

Tension Firing After After After strength temper- 100 1 .000 betweenTest ature Porosity lnitial hours hours hours electrodes N0. (C) 7:)(KO) (KQ) (K0) (K9) (Kg) 29 1,470 55 44 37 38 88 0.4 30 1,485 50 28 2728 34 0.6 31 1,500 45 22 21 22 26 0.8 32 1,515 40 19 18 18 21 1.0 331,535 35 17 17 17 18 1.2 34 1.550 30 15 16 16 1.4 35 1,570 25 14 14 1516 1.6 36 1,600 13 14 15 16 1.8 37 1,620 15 13 13 14 16 1.9 38 1,630 1214 14 15 17 2.0 39 1,645 10 14 15 15 17 2.1 40 1,665 8 14 15 15 17 2.141 1,690 5 12 8 5 1 2.2 42 1,710 3 10 0.1 run 2.2

away

EXAMPLE 2 EXAMPLES 3 AND 4 70 mol percent of a mixture of 12 mol percentof CaO and 88 mol percent of ZrO was added with mol percent of spinelcomposed of MgO and A1 0,; as a resistance controlling agent. Theresulting mixture was calcined at 1,350C for 2 hours and then added with4 percent by weight of an emulsion consisting of equal weight amounts ofstearic acid and water and pulverized in a wet process in a trommel for20 hours. The pulverized mixture was treated in the same manner asdescribed in Example 1 except that the heating temperature lower than1,700C to produce the oxygen ion transport type thermistors. DC 12V wasapplied to each of the thermistors at 800C. The obtained results areshown in the following Table 4. As seen from this table, the thermistorhaving a porosity of 5 percent obtained by sintering at a temperature of1,670C showed run away after 100 hours and was not able to bepractically used.

Table 4 A mixture of 90 mol percent of ZrO and 10 mol percent of Y O anda mixture of 90 mol percent of ThO and 10 mol percent of Y O werecalcined at 1,400C for 2 hours respectively. Then each of the mixtureswas added with 3.2 percent by weight of an emulsion consisting of equalweight amounts of stearic acid and water. The resulting mixture waspulverized in a wet process in a trommel for 8 hours. The pulverizedmixture was treated in the same manner as described in Examples 1 and 2at various temperatures as shown in the following Tables 5 and 6 byusing 0.4 mm alloy wires having and consisting of 70 percent of platinumand 30 percent of rhodium as a pair of electrodes to produce oxygen iontransport type thermistors.

DC 12V was applied to the resulting thermistors and the results as shownin the following Tables 5 and 6 were obtained.

70 moF/z (12 mol7l Ca0+88 mol?! ZrO )+30 mow (MgAl O Resistance 800C,12V

Tension Firing After After After strength tern per- 10 100 1,000 betweenTest ature Porosity Initial hours hours hours electrodes g) 43 1,420 5516 13 14 24 0.5 44 1,465 50 11 9.8 10 14 0.7 45 1,475 45 8.2 7.6 7.8 9.60.9 46 1,505 40 6.4 6.0 6.2 7.3 1.2 47 1,520 35 5.2 5.0 5.1 5.8 1.4 481,545 30 4.4 4.3 4.4 5.2 1.6 49 1,560 25 4.2 4.3 4.4 4.8 1.7 50 1,575 204.2 4.3 4.4 5.0 2.0 51 1,610 15 4.2 4.3 4.5 5.0 2.1 52 1,620 13 4.3 4.34.6 5.0 2.1 53 1,635 11 4.4 4.4 4.6 5.0 2.1 54 1,660 7 4.4 4.4 4.6 4.92.2 55 1,670 5 4.4 1.6 run 2.2

away 56 1,695 3 2.7 run 2.2

Resistance 800C, 12V

Tension Firing After After After strength temper- 10 100 1,000 betweenTest ature Porosity lnitial hours hours hours electrodes No. (C) (/c)(K) (K0) (KS1) (KS2) (Kg).

57 1,420 60 2.6 2.3 2.5 5.4 0.3 58 1,480 55 2.2 1.9 2.1 3.7 0.5 59 1,52050 1.8 1.7 1.8 2.8 0.7 60 1,540 45 1.6 1.4 1.5 2.2 0.9 61 1,570 40 1.41.2 1.3 1.8 1.1 62 1,590 35 1.2 1.1 1.1 1.5 1.3 63 1,620 30 1.0 1.0 1.01.2 1.5 64 1,640 25 0.90 0.88 0.9 1.0 1.6 I 65 1,675 20 0.86 0.85 0.880.98 1.7 66 1,710 15 0.85 0.86 0.89 0.98 1.8 67 1,750 10 0.86 0.86 0.920.98 1.9 68 1,790 7 0.87 0.87 0.92 0.40 2.0 69 1,805 0.87 0.80 0.20 run2.0

away 70 1,820 3 0.70 run 2.1

away

Table 6 90 mol'71 of ThO +l0 mol7r of Y O Resistance 800C, 12V

. v 1 Tension Firing After After After strength temper- 100 1.000between I Test ature Porosity lnitial hours hours hours electrodes No.(C) "/11 K0 (K0) (K0). (K0 (Kg) i 71 1,590 60 72 64 '75 156 -0.4 I 1 721,615 55 65 61 .69 I 112 0.5 I 1 73 1,630 50 58 55 60 71 0.7 i 74 1,65545 44 41 45 56 0.9 1 75 1,670 40 36 34 1.1 76 1,695 35 i 27 4 26 27 301.3 77 1,720 '30 22 22 23 26 1.5 78 1.730 25 20 20 21 24 1.8 79 1,745 20l9 19 20 22 2.0 80 1,760 15 17 18 18 20 2.0 '8! 1,780 11 16 17 17 19 2.182 1,805 8 15 17 17' 17. 2.1 83 1,840 5 14 16 13 run 2.2

away 84 1,870 2 14 7 run 2.2 laway present invention as mentioned above,there is no fear A that the electrodes will fall from the ceramicresistors,

oxygen can be transported freely between both the electrodes, thevariation of the resistance due to the reduction of the ceramic resistorcan be prevented, the' 1. An oxygen ion transport type thermistorcomprising an oxygen ion 'type solid solution consisting of 5 O9 5 molpercent of at least one metal oxide selected from the group consistingof ZrO CeO HfO and ThO and 5-50 mol'percent of at least one metal oxideselected from the group consisting of CaO, MgO, SrO, La O 1 0 Yb O S0 0Gd O ancl Nd O and two metal lead wires are embedded in said solutionand held in an adequately spaced parallel relationship to each other andlying substantially in the center line in the thickness direction ofsaid solution and that the porosity of said oxygen ion transport typesolid solution is 7 percent. v

2 The thermistor as claimed in claim 1, wherein said porosity is 5-35percent.

1. AN OXYGEN ION TRANSPORT TYPE THERMISTOR COMPRISING AN OXYGEN IONSOLID SOLUTION CONSISTING OF 50-95 MOL PERCENT OF AT LEAST ONE METALOXIDE SELECTED FROM THE GROUP CONSISTING OF ZRO2, CEO2, HFO2 AND THO2,AND 5-50 MOL PERCENT OF AT LEAST ONE METAL OXIDE SELECTED FROM THE GROUPCONSISTING OF CAO, MGO, SRO, LA2O3, Y2O3, YB2O3, SC2O3, GD2O3 AND ND2O3,AND TWO METAL LEAD WIRES ARE EMBEDDED IN SAID SOLUTION AND HELD IN ANADEQUATELY SPACED PARALLEL RELATIONSHIP TO EACH OTHER AND LYINGSUBSTANTIALLY IN THE CENTER LINE IN THE THICKNESS DIRECTION OF SAIDSOLUTION AND THAT THE POROSITY OF SAID OXYGEN ION TRANSPORT TYPE SOLIDSOLUTION IS 7-55 PERCENT.
 2. The thermistor as claimed in claim 1,wherein said porosity is 15-35 percent.